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Abstract

Background

Consumption of high calorie junk foods has increased recently, especially among young
adults and higher intake may cause weight gain. There is a need to develop public
health approaches to motivate people to reduce their intake of junk food.

Objective

To assess the effect of health and social norm messages on high calorie snack food
intake (a type of junk food) as a function of usual intake of junk food.

Design

In a between-subjects design, 129 young adults (45 men and 84 women, mean age = 22.4 years,
SD = 4.5) were assigned to one of three conditions: 1) a social norm condition, in
which participants saw a message about the junk food eating habits of others; 2) a
health condition, in which participants saw a message outlining the health benefits
of reducing junk food consumption and; 3) a control condition, in which participants
saw a non-food related message. After exposure to the poster messages, participants
consumed a snack and the choice and amount of snack food consumed was examined covertly.
We also examined whether usual intake of junk food moderated the effect of message
type on high calorie snack food intake.

Results

The amount of high calorie snack food consumed was significantly lower in both the
health and the social norm message condition compared with the control message condition
(36% and 28%, both p < 0.05). There was no significant difference in snack food or
energy intake between the health and social norm message conditions. There was no
evidence that the effect of the messages depended upon usual consumption of junk food.

Conclusions

Messages about the health effects of junk food and social normative messages about
intake of junk food can motivate people to reduce their consumption of high calorie
snack food.

Keywords:

Social norms; Health messages; Junk food

Introduction

There has been a sharp increase in the availability and consumption of junk food in
recent years and this may have contributed to rising rates of obesity [1,2]. Junk foods are defined as items that are high in energy content, fat and/or sugar
and low in nutrients [3]. Consumption of junk food is of concern among young adults because they are a demographic
identified as high consumers of junk food [4,5]. In one study, junk food intake was shown to account for 20% of variance in weight
gain amongst young adults during a 3 month period [3].

There is mixed evidence that nutritional education and point of choice nutritional
labelling encourage choice of ‘healthier’ foods over higher calorie ‘unhealthy’ options,
such as junk food [6,7]. A large number of healthy eating approaches have been centred around increasing
fruit and vegetable consumption [8,9] and public health messages about the health benefits of eating fruit and vegetables
have had some success in increasing consumption [10,11]. The types of messages that motivate people to reduce junk food intake have received
less attention. There are distinct differences between increasing fruit and vegetable
intake and limiting junk food intake in terms of both the behavior (adding to the
diet vs. limiting) and the foods (junk foods are more palatable). Thus, there is a
need to develop an evidence base for message types that will motivate people to reduce
their intake of high calorie junk foods.

In young adults, intake of high calorie snack food has been reported to be influenced
strongly by perceived social norms, as the extent to which people believe others are
consuming large or small amounts of high calorie snack food affects intake [12] (although see Salvy et al. [13] for other types of social influence on eating behaviour). Experimental studies have
shown that participants eat greater amounts of high calorie snack food if they believe
that previous participants in the study have eaten a large amount of snacks than if
they believe the other participants have eaten a small amount [12,14]. These early studies made use of a ‘remote confederate’ design, whereby participants
learn about previous participants either consuming small or large amounts of food.
These findings are of interest, as participants eat alone and are therefore likely
to be changing their behaviour as a result of the information they are exposed to,
not just because they want to make a good impression or be liked by a dining companion.
Similar effects have been observed in children, whereby beliefs about what other children
have been eating have been shown to influence how much food both healthy weight and
overweight children eat [15]. Another interesting observation is that being led to believe others have eaten very
little food produces just as strong an effect on food intake as does eating with another
person who has been instructed to eat very little [16]. Thus, it appears that social norms exert a substantial influence on food intake.
Similar effects have also been reported on food choice. One study [17] found that if participants believe the norm is to select a high calorie snack food,
they are more likely to make similar food selections. Finally, outside of the laboratory,
beliefs about snack amounts consumed by peers are a significant predictor of snack
food intake [18,19].

There is preliminary evidence that social norm messages can improve the healthiness
of dietary choices [20,21]. An intervention study by Stok and colleagues [20] found that repeated exposure to social norms about fruit intake resulted in participants
self-reporting increased fruit consumption. In a recent study we found that a social
norm message was more effective than a health message at increasing the actual consumption
of fruit and vegetables in a laboratory test meal [22]. The effect was moderated by participants’ usual intake of fruit and vegetables,
whereby usually low consumers increased their intake, but usually high consumers did
not. We hypothesised that this was because the low consumers felt motivated to change
their behavior to adhere to the social norm message, while the high consumers were
already doing so.

The use of social norm messages to motivate reductions in junk food consumption has
not been examined. A novel hypothesis is that messages outlining that other people
limit their junk food intake (a social norm message) may motivate individuals to reduce
the amount of junk food they eat. As people are motivated to conform to social group
norms [23], we might expect that high consumers of junk food would be more susceptible to norm
messages. This is because high consumers of junk food are likely to perceive that
they are out of line with the presented norm and adjust their behaviour accordingly,
whereas low consumers of junk food will already be in line with the norm and so will
be less likely to change their behaviour [23].

In the present laboratory study, we examined whether a health message and a social
norm message about limiting junk food intake would motivate people to reduce their
intake of high calorie snack food (a type of junk food). We exposed young adults to
a message on a poster and observed their later food choices and consumption of both
high calorie snack food and fruit and vegetables. By offering a choice of foods we
were able to assess whether messages reduced junk food intake and whether this would
also cause a corresponding reduction in total amount of energy consumed when alternative
foods were available in their place. In line with research on healthy eating promotion
[10,11], we hypothesised that participants in the health message condition would consume
less high calorie snack food intake than participants in the no intervention control
message condition. Based on our recent findings regarding the effect of social norm
messages on fruit and vegetable consumption [22] we hypothesised that the social norms message about junk food intake may have a larger
effect than the health message. We also predicted that the effect of message type
may be moderated by usual junk food intake, whereby high consumers of junk food would
be most influenced by the social norms message.

Method

Design

A 3 × 2 between-subjects design was used, with factors: message type (social norm/health
/control) and usual junk food intake (low consumers/high consumers). Prior to taking
part in what they believed was a mood and eating study, participants evaluated a poster
containing a social norm, health or control message. Participants later completed
mock mood measures before selecting and consuming a snack in a naturalistic setting.

Participants

Sample size was determined using power calculations (GPower 3.1) on data from two
recent studies that compared the use of social norm and health messages on fruit and
vegetable intake in our study population [19]. At 80% power, α = 0.05, f = 0.28, a sample size of 127 participants was estimated
to detect significant main and interaction effects. We aimed to recruit slightly above
this number to account for any participants providing incomplete data sets.

One-hundred and twenty nine adults (45 men and 86 women) with a mean age of 22.4 years
(SD = 4.5) were recruited through online and campus advertisement. The adverts suggested
the study was about the effect that eating has on mood. Participation was in exchange
for £5 Sterling or course credit. Participants were required to abstain from eating
during the two hours prior to their session and could not have any food allergies.
All participants that responded to advertisement were eligible. Recruitment took place
between July 2012 and August 2012 at the University of Birmingham. The study was conducted
according to the guidelines laid down in the Declaration of Helsinki and all procedures
were approved by the University of Birmingham Research Ethics Committee. Participants
gave written informed consent. We randomized participants through a simple random
allocation sequence using researchrandomizer.org.

Messages

Prior to arrival participants were randomly assigned to one of the three conditions.
In the social norm and health conditions participants viewed a poster containing images
of junk food (a hamburger, fries, soda, candy wrappers) and a message defining junk
food: ‘junk food is high calorie food with low nutritional value’. The posters only
differed in the content of a message in the middle of the poster. In the social norm
condition this read ‘Students eat less junk food than you might realise. Most students limit how much junk
food they are eating to 1 or less than 1 serving a day.* based on a 2012 study. This statistic was derived from a pilot survey study that we conducted with 40 students.
The pilot study was an internet survey conducted shortly before data collection for
the main study. Participants answered questions similar to the self-report measures
described in the section ‘Measurements’. The aim was to formulate a norm message based
on junk food intake in the target population.

In the health condition the message read ‘Reducing junk food intake is good for your health. Limiting junk food to 1 or less
than 1 serving a day is part of a healthy diet. * based on a 2012 study’. In the control condition the poster was of a similar word length to the two other
conditions, but the message emphasised the importance of preparing in advance for
exams. The images were of textbooks. There was no reference to junk food in the poster.

Food

The snack buffet consisted of 6 common food items in the UK. Three of these were high
calorie snack foods; chocolate chip cookies (481 kcal/100 g), ready salted crisps
(537 kcal /100 g) and chocolate finger biscuits (515 kcal/100 g). We selected these
foods because participants in our pilot study reported that they ate these foods often
and also that they were widely perceived as ‘junk food’. The fruit and vegetable items
were carrot sticks (64 kcal/100 g), sliced apple (53 kcal/100 g) and grapes (64 kcal/100 g).
All foods were purchased from Sainsbury’s Ltd (United Kingdom) and presented on individual
plates. The participants were not made aware that we were examining food choice.

Measurements

Baseline hunger: To assess baseline hunger the participants used a 10 cm line scale ‘how hungry are
you right now? (mark with an x)’ with anchors ‘not at all’ and ‘extremely’. Mock mood questionnaire: To corroborate the cover story, the participants rated various moods (e.g. how happy, tired, hungry, anxious, alert, angry, stressed are you?) on separate 10 cm line scales, with anchors ‘not at all’ and ‘extremely’.
Cognitive restraint scale: To assess dieting tendencies, all participants completed the cognitive restraint
scale of the Three Factor Eating Questionnaire [24]. Usual junk food intake measure: To assess usual intake of junk food of participants we included two items. The first
was a guided one day dietary recall measure [25]. Participants were asked to ‘think back carefully, working from when you woke up,
please list all of the food items you ate yesterday’. Participants were then asked
how many of the recalled items were junk food. We opted for participant classification
(rather than a researcher coding this), as our hypothesis was that it would be participants’
perceptions of how much junk food they eat that would moderate the influence of a norm message.
The second measure was of usual consumption. Participants were asked to indicate how
many servings of junk food they normally ate a day. We did not provide explicit instruction
about the types of food they should class as ‘junk food’. Manipulation check: To check whether participants classed the high calorie foods we included in the buffet
as ‘junk food’ we asked participants to list the buffet items they classed as junk
food. To check participants had correctly read the content of the poster, we asked
participants to write down the message content.

Procedure

Experimental sessions took place in the School of Psychology, University of Birmingham,
UK. The sessions took place between 10 am-12 pm and 2-5 pm on weekdays. The researcher
greeted the participant and explained they would be taking part in a mood study. The
researcher also explained that another research group had developed some posters and
because the mood study would not take long, it would be appreciated if the participant
could provide some feedback on a poster first. After signing for consent for the poster
study and providing demographic information, the participant was seated alone in a
room and provided with the poster, which they rated for clarity and the extent to
which they understood the messages (using a five point likert scale response format).
The participant was then taken to another room to start the mood study and the researcher
explained the cover story; the effect of eating on mood. The participant signed for
consent, completed the baseline hunger measure and then the mock mood questionnaire.
The participant was taken to a kitchen area where a naturalistic snack buffet was
set out. After being provided with a plate, the participant was instructed to select
whatever they wanted. Food selection and consumption occurred alone after which the
researcher returned and gave the participant another mock mood questionnaire (to back
up the cover story) and the cognitive restraint scale to complete. The participant
was then asked to write down the aims of the study before having their height and
weight measured using a stadiometer and digital scales to allow calculation of body
mass index (BMI, kg/metres2). Finally, the participant completed the usual junk food intake measures and the
manipulation checks. Participants were thanked and debriefed. The buffet foods were
then weighed to calculate selections and any food selected but not consumed was also
weighed.

Strategy of analysis

We planned one way ANOVAs on baseline variables: hunger; BMI; restraint; and age,
in order to check whether there measures differed according to condition. If one or
more variables were found not to be balanced across conditions and correlated with
the main dependent variable (grams of high calorie snack food consumed), we controlled
for this using covariates in subsequent analyses.

Our main planned analysis strategy was to use a between-subjects 2×3 ANOVA to compare
the independent and interaction effect of condition and usual intake on our main dependent
measure: grams of high calorie snack food consumed. We planned the same strategy to
examine the effect of condition and usual intake of junk food on total kcals consumed.
This was to determine whether any differences in high calorie snack food intake resulted
in an overall reduction in energy intake.

Results

Manipulation checks & demand awareness

All participants completed the experiment and were included in analyses. All participants
were able to recall the message they had earlier read. Participants also categorised
the three high calorie snack foods from the buffet as junk foods, with 123/129 identifying
all three as junk foods. There was no evidence of demand awareness, in that participants
were not aware of the true study aims. The cover story was widely believed. Of the
129 participants, only six mentioned that posters and eating study might have been
linked. Removal of their data did not affect the results.

Usual junk food intake

The two items measuring usual junk food intake were significantly correlated (r = 0.57,
p < 0.05), so we averaged them to form an average measurement of usual junk food intake.
A median split on the average measurement resulted in 67 participants being classed
as high consumers of junk food (mean daily portions = 2.5, SD = 0.9) and 62 participants
as being classed as low consumers of junk food (mean daily portions = 0.9, SD = 0.4).

Baseline variables

The mean BMI of the sample was 23.2 (SD = 3.9, range = 16.1-41.4), which is within
the healthy weight range of 18.5 – 24.9. The mean restraint score of the sample = 8.4
/ 21, SD = 5.4, range = 0–21 (cronbach alpha = 0.88), suggesting that dieting tendencies
were not high in the sample. The restraint scale ranges from 0–21 and the mean of
this sample is similar to other studies sampling this population [22,25]. Mean baseline hunger score = 5.6 cm / 10.0 cm (SD = 2.2, range = 0.2-10.0). Hunger
was marginally lower (p = 0.05) in the health message condition than the social norm
message and control message conditions. See Table 1 for ANOVA results. Because of this we tested where baseline hunger was significantly
correlated with junk food intake (r = 0.34, p <0.05). As baseline hunger was correlated
with junk food intake we included baseline hunger as a covariate in subsequent analyses.

Food choice & intake

Participants tended to eat all of the food they selected (121 participants completely
cleaned their plates). The remaining 6% only left a small amount of food, which totalled
less than 20 grams of food, on average. The exact same pattern of results is obtained
for food choice and food intake, so we only report analyses for food intake. See Table 2.

Next we wanted to follow up the effect of condition. As the results indicated that
hunger was not balanced across conditions and usual junk food intake was associated
with amount of high calorie snack food consumed, we wanted to control for these variables
when assessing the effect of condition. To do this, we conducted linear regression
analysis. We used dummy coding to assess the impact of both the social norm condition
and health condition vs. the control condition on grams of high calorie snack food,
whilst controlling for baseline hunger and usual junk food intake, by including them
in the model. Both the social norm condition [B = − 11.5, p = 0.046] and the health
condition [B = −14.7, p = 0.01] significantly predicted grams of junk food, whereby
both were associated with reduced junk food consumption compared with the control
condition. We also tested if the social norm and health condition differed in the
effect they had on grams of junk food and they did not (p = 0.32). See Table 2.

In order to test whether condition differences in junk food intake resulted in differences
in fruit and vegetable intake we also used the same analysis strategy to examine whether
there were group differences for fruit and vegetable consumption. There was no main
effect of condition in the 2×3 ANCOVA [F(2, 122) = .0.57, p = 0.57, ηp2 = 0.009] on grams of fruit and vegetables consumed. In conjunction with the results
for total calories consumed this result suggests that participants in the social norm
and health message conditions were not replacing junk food items with fruit and vegetables.
See Table 2.

Discussion

We examined whether messages about junk food intake would motivate individuals to
decrease their consumption of high calorie snack food. Viewing messages outlining
the health benefits of limiting junk food intake and that a social norm is to limit
junk food intake was associated with less high calorie snack food intake than viewing
a control message. There was no evidence that the social norm message had a greater
effect than the health message. Compared with the control message, participants in
the experimental conditions ate less high calorie snack food (32% on average, which
equated to 70 fewer calories in a single snack session). These findings are promising,
given that healthy eating interventions typically only promote modest changes to dietary
behavior [10]. Although our measure of usual intake of junk food predicted the amount of high calorie
snack food participants ate (usually high consumers ate more junk food), there was
no evidence of an interaction between this measure and message type. Thus, the messages
had similar effects on high calorie snack food intake across both high and low consumers
of junk food. Total energy intake was also lower in the experimental conditions versus
control condition, suggesting that the messages reduced junk food intake and there
was little evidence of compensation or substitution with other food items.

Developing public health messages that encourage people to limit their consumption
of high calorie foods is a priority, as the widespread availability and intake of
these foods is thought to be a significant contributor to rises in adiposity [1,3]. Although much research has tested the types of interventions that could encourage
people to increase their intake of fruit and vegetables, messages specifically targeting
junk foods have received less attention [8,10]. The present study suggests two message types that may be effective. The use of social
norm messages to encourage healthier eating is a novel concept but evidence has started
to accumulate that they could be used to promote fruit and vegetable intake [20-22]. This is the first study to test the effectiveness of social norm message in reducing
intake of junk food.

In a previous study, we observed that social norm messages about consumption of fruit
and vegetables were associated with greater fruit and vegetable intake in individuals
who were usually low consumers, but not high consumers. This is likely to be because
high consumers were already adhering to the norm, whilst low consumers felt motivated
to increase their intake to adhere to the presented norm [22]. See Schultz et al. for a discussion of how norms influence behaviour as a function
of whether individuals believe they are adhering to a presented norm [23]. In the present study, we predicted that usual intake of junk food would moderate
the effect of a social norm message on intake of high calorie snack food/junk food.
There was no evidence for such moderation in the present study; lower junk food intake
was associated with exposure to the experimental messages regardless of usual intake.
One explanation for this finding may be that the low usual consumers of junk food
did not perceive themselves to be adhering to the norm we presented. The norm message
was that ‘others limit their junk food intake to 0 or 1 portion a day’ and very few
participants reported normally consuming 0 portions a day. Thus, it could have been
that all participants were motivated to adhere to the norm. Measuring participant
perceptions of how their current behavior relates to a norm presented and testing
the influence this has on junk food intake would allow for a more direct examination
of this proposition in future studies.

Previously, we had also observed that social norm messages were more effective than
health messages for promotion of fruit and vegetable intake [22]. A similar pattern of results has been found when measuring intentions to consume
fruit and vegetables [21]. This was not the case in the present study as participants in both experimental
conditions ate less high calorie snack food intake than the control condition. It
is possible that health messages about fruit and vegetable intake are now commonplace
[9,11], so the health implications of eating fruit and vegetables are already well understood.
Conversely, messages about the health implications of consuming high amounts of junk
food may not be as commonly understood in this population, so individuals may be less
well informed. This suggestion is speculative, so evidence will be needed to confirm
it. The message types assessed here are likely to bring about changes to eating behavior
through different mechanisms (social motives vs. health motivates), so it would be
interesting to examine whether an additive effect would be observed on combining messages.
Examination of individual differences in responsiveness to message types would also
be valuable. Health messages might be particularly influential on people with high
health concern but they may have little impact on people with low health concern [26,27], whereas social norm messages may motivate both sets of people.

A strength of the present study was the inclusion of a control condition, in which
participants were not exposed to food messages. This allowed us to make a direct comparison
between intervention messages and a non-intervention message. In future studies it
would be informative to test the effects of a message that instructs participants
to reduce their junk food intake, without specific reference to health motives or
social norms, as this would allow for a clearer examination of the importance of food-related
message content. Eating behavior was measured shortly after exposure to messages,
so although messages may have motivated participants to change their behavior in a
healthy way, whether these changes would be maintained over longer periods requires
examination. Social norm messages to promote other health behaviors have produced
longer lasting effects, so longer term changes might occur [28]. As we tested participants in a lab environment and instructed them to read poster
messages, it will be important in the future to examine whether these findings can
be generalized to applied ‘real world’ settings. It will also be important to examine
the effect of different social norm reference groups [29]. Here, young adult students participated and the social norm message was about what
other students were eating. Thus, although this type of approach could be effective in university
campus settings, further evidence will be needed to assess whether more encompassing
norm messages would motivate healthy changes to dietary behavior and if effects would
occur in other subsections of the population. That being said, some recent work has
suggested messages about national eating norms can increase peoples intentions to
eat healthily in the general population [21], so this deserves further attention. Examining whether social norms approaches could
be used to promote healthy eating in young children also warrants attention, as Salvy
and colleagues in particular have shown peers to have a substantial influence on snack
food intake in a number of studies in this population [13,30].

The results of the present study suggest that participants did not compensate for
their reduction in junk food by eating more of the fruit and vegetables that were
also on offer. It may be the case that if other more palatable non-‘junk foods’ were
available, participants would have compensated by eating these foods. Further work
will need to clarify this. In line with other research examining social norm effects,
we used self-report measures of dietary behavior [20,22]. Although we included a guided dietary recall and a measure of typical consumption,
further work would benefit from measures less likely to be influenced by reporting
bias.

The results from the present study show that messages about the health benefits and
social norms surrounding ‘junk food’ intake are associated with reduced intake of
high calorie snack food in young adults. The results also contribute to a new body
of evidence that suggests social norm based messages can motivate people to eat more
healthily.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

ER, EH, JT, PA and SH conceived the study and participated in its design. EH and JT
organised the co-ordination of the study. EH and ER conducted the research and analysed
the data. ER, PA and SH were responsible for the first drafting of the manuscript
and all authors approved the final manuscript.

Acknowledgements

The authors would like to thank Katy Smart and Victoria Whitelock for their research
assistance. EH was funded by the University of Birmingham Centre for Obesity Research.
PA was funded by The UK Centre for Tobacco Control Studies, a UKCRC Public Health
Research: Centre of Excellence. Funding from British Heart Foundation, Cancer Research
UK, Economic and Social Research Council, Medical Research Council, and the Department
of Health, under the auspices of the UK Clinical Research Collaboration, is gratefully
acknowledged.